The present invention relates to an optical fiber, particularly, to an optical transmission line suitably used in a wavelength division multiplexing (WDM) optical communications.
The WDM transmission is a system in which a signal having a plurality of wavelengths is transmitted by a single optical fiber. In recent years, an optical amplifier using an erbium-doped optical fiber (EDFA) has been developed, making it unnecessary to convert an optical signal into an electric signal for each wavelength in the relay by the WDM transmission. The advent of the EDFA accelerates the inclination toward the WDM transmission.
Large factors inhibiting the high speed transmission of optical signals include a chromatic dispersion and the nonlinear properties. If the chromatic dispersion is large, the deterioration of the waveform proceeds so as to make it impossible to perform a high speed transmission. On the other hand, if the chromatic dispersion approaches zero, the four wave mixing (FWM), which is one of the nonlinear phenomena, is generated so as to make it difficult to perform the WDM transmission.
For avoiding the phenomenon, proposed in Japanese Patent Disclosure (Kokai) No. 7-168046 is an optical fiber whose nonlinear phenomenon was suppressed so as to permit the optical fiber to bear a non-zero dispersion in the transmission band of the optical signal.
The problems of dispersion and nonlinear properties seemed to have been solved by the use of the optical fiber for the WDM transmission. If the number of optical signals used for the WDM transmission is increased to meet the demands for the large capacity transmission, it is necessary to maintain a non-zero dispersion over a wide wavelength band. However, the dispersion slope of the optical fiber for the WDM transmission makes it impossible to maintain a non-zero dispersion over a wide wavelength band.
It may be possible to realize an optical fiber having a non-zero dispersion over the entire region of the transmission band and having a dispersion slope that is substantially zero. However, the optical fiber of this type is generally poor in the nonlinear properties. If the optical fiber of this type is connected immediately after the optical amplifier, distortion is generated in the optical signal.
Further, where a large capacity transmission is performed at a high speed, it is desirable to diminish the accumulated dispersion, which is the chromatic dispersion over the entire optical transmission line, as much as possible. However, where the optical transmission line is formed of only the optical fibers for the WDM transmission, the chromatic dispersion of the optical fiber approaches zero, with the result that FWM tends to be generated.
An object of the present invention is to provide an optical fiber that permits compensating the dispersion slope of an optical transmission line using optical fibers having a non-zero dispersion in the transmission band of optical signals so as to make it possible to achieve the WDM transmission over a wide transmission band.
Another object of the present invention is to provide a dispersion compensator using the particular optical fiber.
Another object of the present invention is to provide an optical transmission line using the particular optical fiber.
Still another object of the present invention is to provide an optical transmission system using the particular optical fiber.
According to a first aspect of the present invention, there is provided an optical fiber which comprises a core and a two-layer clad including an innermost layer and an outermost layer, and has a refractive index distribution structure satisfying the conditions of 0.8%xe2x89xa6xcex941xe2x89xa61.3% and xe2x88x920.7%xe2x89xa6xcex942xe2x89xa6xe2x88x920.4%, where xcex941 represents a relative refractive index difference of the core with respect to the outermost layer of the clad, and xcex942 represents a relative refractive index difference of the innermost layer of the clad with respect to the outermost layer of the clad, and also satisfying the conditions of xe2x88x9220xe2x89xa6Dxe2x89xa60, xe2x88x920.1xe2x89xa6S less than 0 and 0xe2x89xa6(D/S)xe2x89xa6200 in specified wavelength band having a band width of at least 20 nm in a 1.5 xcexcm wavelength band, where D represents the chromatic dispersion (ps/nm/km) at a specified wavelength in a 1.5 xcexcm wavelength band, and S represents the dispersion slope (ps/nm2/km).
It is desirable for the optical fiber of the present invention to satisfy the conditions of 2xe2x89xa6axe2x89xa65, axe2x89xa6bxe2x89xa615 and 0.3xe2x89xa6(a/b)xe2x89xa60.5, where a represents the outer diameter of the core, and b represents the outer diameter of the innermost layer of the clad.
According to a second aspect of the present invention, there is provided a dispersion compensator comprising the optical fiber defined as above and capable of compensating an average dispersion slope (ps/sm2/km) to be not smaller than xe2x88x920.01 and not larger than 0.01 in specified wavelength band having a band width of at least 20 nm in a 1.5 xcexcm wavelength band, when the dispersion compensator is incorporated in an optical transmission line.
When the dispersion compensator is incorporated in an optical transmission line, it is desirable for the dispersion compensator to be capable of compensating an average dispersion slope (ps/nm2/km) to be not smaller than xe2x88x920.03 and not larger than 0.03 in specified wavelength band having a band width of at least 40 nm in a 1.5 xcexcm wavelength band.
According to a third embodiment of the present invention, there is provided an optical transmission line comprising the optical fiber defined as above, wherein an average dispersion slope (ps/nm2/km) in specified wavelength band having a band width of at least 20 nm in a 1.5 xcexcm wavelength band is compensated to be not smaller than xe2x88x920.01 and not larger than 0.01.
In the optical transmission line of the present invention, it is desirable for an average dispersion slope (ps/nm2/km) in specified wavelength band having a band width of at least 40 nm in a 1.5 xcexcm wavelength band to be compensated to be not smaller than xe2x88x920.03 and not larger than 0.03.
According to a fourth aspect of the present invention, there is provided an optical transmission system comprising the optical fiber defined as above, wherein an average dispersion slope (ps/nm2/km) in specified wavelength band having a band width of at least 20 nm in a 1.5 xcexcm wavelength band is compensated to be not smaller than xe2x88x920.01 and not larger than 0.01.
In the optical transmission system of the present invention, it is desirable for an average dispersion slope (ps/nm2/km) in specified wavelength band having a band width of at least 40 nm in a 1.5 xcexcm wavelength band to be compensated to be not smaller than xe2x88x920.03 and not larger than 0.03.
The optical transmission system according to the fourth aspect of the present invention can be realized suitably by the dispersion compensator according to the second aspect of the present invention and by the optical transmission line according to the third aspect of the present invention.
The dispersion value of the optical fiber used in the WDM transmission line is about 2 to 6 ps/nm/km, and the average dispersion slope is about +0.07 ps/nm2/km. An optical transmission line capable of compensating the non-zero dispersion, decreasing the dispersion slope, and exhibiting flat dispersion characteristics over a wide wavelength range can be realized by connecting an optical fiber having a negative chromatic dispersion and a negative dispersion slope to a transmission line having a non-zero dispersion and a dispersion slope. As a result, the wavelength band that can be used can be widened, making it possible to achieve a WDM transmission over a wide wavelength band.
In each of the aspects described above, the expression xe2x80x9c1.5 xcexcm wavelength bandxe2x80x9d represents a band having a wavelength of 1520 to 1620 nm unless otherwise specified herein later. Likewise, the xe2x80x9cspecified wavelength in a 1.5 xcexcm wavelength bandxe2x80x9d represents a specified wavelength such as 1550 nm. Further, the xe2x80x9cspecified wavelength band in a 1.5 xcexcm wavelength bandxe2x80x9d falls within the band of 1520 to 1620 nm and represents a wavelength band under which the optical transmission is actually performed in the optical transmission line, e.g., the conventional 1.55 xcexcm wavelength band (which represents the wavelength of 1530 to 1570 nm in many cases).
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.